Think of adding 1000lbs to a 5000lb boat (20%) increase compared to 1000lbs to a 15,000lb boat 6.6% increase, it is even more extreme when you figure inultra light weight and really heavy disp.
I am aware of that. But as I said, if the hull is already at the design max, it doesn't really matter at that point. I also used the example of an old design, but made from lighter and stronger materials, allowing it to carry more, and yet be well under the weight of the original. The point I was making is that a boat of any given weight can be designed to carry weight. In theory you can build a huge tub (literally) from carbon fiber and it will carry a lot. Now, if you let the sides of the tub bulge outwards from the designed waterline and up (flare, basically), you will have the most resistance to being emerged with extra weight.
How do you know that old hull is not already overweight when it's empty? Most "old school" boats are overbuilt to the extreme, and they're old, so they probably have soaked a lot water since then. Of course, that's even without considering how much weight all that extra surface there is on an old hull, compared to a modern fin keeler or the like, also adding to the weight.
An not all heavy disp boat can't get out of there own way in anything short of a gale. I don't know what it takes to get my boat to hull speed, but main and working jib will do the trip in less than 10kts.
Hull speed is funny thing: More modern designs surpass the hull speed of older designs at the same length. Having more weight and more surface area under water (drag, in other words), also makes it more resistant to light breezes. You can design a lightweight boat, with a more modern hull form (and keel and rudder etc) that can carry at least as much weight as your old thing, and since the all-up weight will still be lower than the old design, it will perform better.
The same hull design of that heavy displacement cruiser would be a much better performer if it were lighter overall. It would be able to carry more, and still come in lighter than it does now.
I'm not saying that all old boats are slow, but if one of the most important criteria is that it should be considered a cargo vessel and carry 1 or 2 tons of extra gear, obviously the design will reflect that and as a consequence tend to be rather slow in light winds. Not to mention that a lot of cruisers don't like to do too much "tweaking", so they tend to go with a conservative sail area, making it even worse.